The present invention relates generally to gas turbine engines, and, more specifically to combustors therein. In a gas turbine engine, air is pressurized in a compressor and channeled to a combustor, mixed with fuel, and ignited for generating hot combustion gases which flow downstream through one or more turbine stages. In a turbofan engine, a high pressure turbine drives the compressor, and is followed in turn by a low pressure turbine which drives a fan disposed upstream of the compressor.
A typical combustor is annular and axisymmetrical about the longitudinal axial centerline axis of the engine, and includes a radially outer combustion liner and radially inner combustion liner joined at upstream ends thereof to a combustor dome. Mounted in the dome are a plurality of circumferentially spaced apart carburetors each including an air swirler and a center fuel injector. Fuel is mixed with the compressed air from the compressor and ignited for generating the hot combustion gases which flow downstream through the combustor and in turn through the high and low pressure turbines which extract energy therefrom.
A major portion of the compressor air is mixed with the fuel in the combustor for generating the combustion gases. Another portion of the compressor air is channeled externally or outboard of the combustor for use in cooling the combustion liners. Another portion of the compressor air is channeled radially through the combustion liner as a jet of dilution air which both reduces the temperature of the combustion gases exiting the combustor and controls the circumferential and radial temperature profiles thereof for optimum performance of the turbines.
A combustor is typically cooled by establishing a cooling film of the compressor air in a substantially continuous boundary layer or air blanket along the inner or inboard surfaces of the combustion liners which confine the combustion gases therein. The film cooling layer provides an effective barrier between the metallic combustion liners and the hot combustion gases for protecting the liners against the heat thereof and ensuring a suitable useful life thereof.
In a typical combustor, the film cooling layer is formed in a plurality of axially spaced apart film cooling nuggets which are annular manifolds fed by a plurality of inlet holes, with a downstream extending annular lip which defines a continuous circumferential outlet slot for discharging the cooling air as a film along the hot side of the liners. The rows of nuggets ensure that the film is axially reenergized from row to row for maintaining a suitably thick boundary layer to protect the liners.
In a recent development in combustor design, a multihole film cooled combustor liner eliminates the conventional nuggets and instead uses a substantially uniform thickness, single sheet metal liner with a dense pattern of multiholes to effect film cooling. The individual multiholes are inclined through the liner at a preferred angle of about 20.degree., with an inlet on the outboard, cold surface of the liner, and an outlet on the inboard, hot surface of the liner spaced axially downstream from the inlet. The diameter of the multiholes is about 20-30 mils (0.51-0.76 mm). This effects a substantially large length to diameter ratio for the multiholes for providing internal convection cooling of the liner therearound. And, most significantly, the small inclination angle allows the discharged cooling air to attach along the inboard surface of the liner to establish the cooling film layer which is fed by the multiple rows of the multiholes to achieve a maximum boundary layer thickness which is reenergized and maintained from row to row in the aft or downstream direction along the combustor liners.
An example of the multihole combustor liner is found in U.S. Pat. No. 5,181,379 assigned to the present assignee, and several additional patents therefor have also issued thereafter. For example, in U.S. Pat. No. 5,261,223, also assigned to the present assignee, an improved multihole combustor liner is disclosed which includes rectangular film restarting holes disposed downstream of the dilution holes. Since the purpose of the dilution holes is to inject substantially large volumes of the compressor air in jets radially into the combustor for controlling the exit gas temperature profiles, the dilution holes inherently interrupt the film cooling layer locally downstream therefrom. Relatively large rectangular film restarting holes are introduced in the combustor liner downstream of the dilution holes and upstream of corresponding ones of the multiholes. The restarting holes are inclined at the same angle, for example 20.degree., as the multiholes for reintroducing the cooling air in attachment along the hot side of the liner.
However, in view of the 20.degree. inclination angle of the multiholes, or the rectangular restarting holes, there remains downstream of the individual dilution holes a dry or shadow region on the hot side of the liner which is inherently devoid of film cooling injection sites. Since the multiholes are inclined downwardly in a downstream direction from the dilution holes, their inlets may be spaced closely adjacent to the downstream portions of the dilution holes, but their outlets are necessarily spaced further downstream from the dilution holes forming the imperforate shadow on the inboard side of the liner downstream of the dilution holes. The multiholes are not allowed to intersect each other or the dilution holes to avoid undesirable stress concentration thereat. The multiholes are typically arranged in uniform patterns, or sub-patterns, for both maximizing the effectiveness of the established cooling film layer as well as ensuring mechanical strength of the liner for obtaining a suitable useful life.
The multihole shadows are acceptable for relatively small secondary holes through the liner such as secondary dilution holes. As the diameter of such secondary holes increases, the corresponding shadow necessarily increases in area, with an attendant higher liner operating temperature which can adversely affect combustor life.
For example, in a further development of multihole combustors for higher thrust engines, the heat loads in the combustor correspondingly increase, which in turn increases the operating temperature in the multihole shadows. The increased temperature decreases the life of the liner which would eventually fail by thermal fatigue cracks in the shadows adjacent to secondary holes.
Accordingly, it is desired to further improve the multihole combustor liner with improved cooling around the secondary holes.